Numerical Simulation Of Seismic Response Of Underground Hydropower Caverns

The Southwest and Northwest of China belong to earthquake-prone areas of high intensity where strong earthquake frequently occurred. There are criss-cross and large scale underground caverns in dozens of large-sized hydropower projects which are as-built, under construction or in stage of planning and design. Because of the large excavation span, high sidewall and complicated structure and stress conditions, stability of rock mass around underground cavern has been closely concerned by engineering professionals all the time. In recent years, when considering seismic and dynamic factors, seismic response of the cavern is becoming hot point and difficult point in the research in our country.This article first briefly described the current situation of study of the dynamic analysis of underground caverns, and then, numerical simulation on seismic response of underground caverns has been carried out by applying explicit lagrangian difference method based on a practical project. Lastly, this article compared and analyzed the seismic response effects of presence or absence of the concrete structure in underground powerhouse. During the simulation, many factors were taken into consideration such as the in-situ stress, excavation by steps, support by cables, material nonlinearity of rock mass and poor geological bodies (described by the Mohr-Coulomb model with a tensile truncation), the time-frequency domain characteristics of displacement, velocity, acceleration, stress of the surrounding rock of the main powerhouse during the earthquake process was also studied. The stability of the surrounding rock mass can be determined by conditions such as distribution of plastic zones and failure approach index (FAI), whether there is mutation of displacement of the key points, whether there is convergence of the unbalanced force and displacement during computation. The main conclusions are as follows:(1) As to the underground rock engineering, the dynamic response of rock belongs to low-frequency seismic vibrations.(2) Assume that the seismic wave propagate upward vertically, when excavation in the underground cavern is in the size range(Span<40m, sidewall height<80m), the waveform of the seismic response of each point along sidewall of the main power house is similar.(phase difference is very small) (3) The dynamic stability of the surrounding rock can be determined according to conditions of whether the plastic zones around the rock hole are connected, whether there is mutation among the displacement of the key points, whether there is convergence of the unbalanced force and the nodes displacement during computation in the earthquake process.(4) When analyzing the seismic stability of underground cavern, we may not take the existence of the internal structure of power house into consideration. The existence of the internal structure reduced the amplitude of the seismic response of the surrounding rock and the distribution of the plastic zones.(5) Bigger section of the underground cavern is harmful to the dynamic stability.This study can provide with ideas and methods on understanding the characteristics of the seismic response of the underground caverns and reviewing their dynamic stability, some conclusions gained can be. used for the design reference.